the rule of thumb is 3.0v per cell on a lithium battery.
any lower than that and you are treading on "irreversible damage" territory.
Drop the voltage to low and the internal gel/acid/whatever solidifies and you have an ugly paperweight.
Lithium
batteries like to stay topped up.
some smart chargers i've owned, have had error displayed when the voltage has reached 3.0v on the cell.
But I find it hard to get to 3.0V on a single battery, as i no longer enjoy the
vape @ 3.6v or below.
My batterries usually get swapped out once i reach that point...